This project includes those projects in which the mass spectrometry workgroup collaborates with other groups, both within and without the Institute to solve problems of mutual interest. The major focuses of these projects are: 1) structure determination of unknown compounds; 2) identification and/or confirmation of biological pathways; 3) quantitation; and 4) development of strategies for the structure determination of biologically important compounds. Current major collaborative projects under way include: 1) identification of ligands for orphan receptors (C. Weinberger, LRDT) and; 2) quantitation of arachidonic acid metabolites (D. Zeldin, LPP). The project descriptions below demonstrate the wide variety of projects in which mass spectrometry can play an integral role. Orphan Receptors - RXR is a nuclear receptor that plays a pivotal role in cell signaling by pairing with receptors for thyroid hormones, retinoids, and vitamin D. Previously, 9-cis-retinoic was identified as a potent RXR activator. However, Weinberger (LRDT) found that a second compound isolated from bovine serum had RXR-specific activator properties. In collaboration with the LRDT group, we have identified this second activator to be the chlorophyll metabolite phytanic acid. Although orders of magnitude less potent than 9-cis-retinoic acid, phytanic acid is found at sufficiently high levels, micro molar, to induce RXR in competition with 9-cis-retinoic acid. We are currently working on the identification of another orphan receptor activator. Quantitation of Arachidonic Acid Metabolites - The most sensitive and accurate method for quantitation of eicosanoids in physiological samples is by selected ion monitoring (SIM) under negative ion chemical ionization (NICI) mass spectrometry. We are currently quantifying epoxy- and dihydroxy-eicosatrienoic acids. The quantitation of epoxyeicosanoids is part of a study identifying formation of these compounds for the first time in the human heart and identification of the protein regulating that formation.